1. Field of the Invention
This invention generally relates to a thermal printhead that is used to print images onto recording paper by means of thermo-sensitive printing or thermal ink-transfer printing.
2. Background Information
FIG. 16 shows an example of a conventional thermal printhead. This thermal printhead includes a common electrode 163, individual electrodes 164, and a belt shaped heating resistor 165, all of which are disposed on top of an insulating substrate 161. The common electrode 163 includes a belt shaped common line 163b, and a plurality of projections 163c that project out from the common line 163b like the teeth of a comb. Each first end 164a of each individual electrode 164 is interposed between two adjacent-projections 163c. Each individual electrode 164 and each projection 163c intersect with the heating resistor 165 and are in electrical contact therewith. Note that, although not shown in the figures, a terminal is formed on one end of the common line 163b of the common electrode 163, and the common electrode 163 is connected to a voltage application means via this terminal. In addition, terminals are formed on second ends of each individual electrode 164 (not shown in the figures). These terminals are each connected to drive IC chips that serve to independently apply a voltage to each individual electrode 164.
In a thermal printhead constructed in this manner, for example, a positive voltage is applied to the common electrode 163 while one individual electrode 164 is connected to ground. An electric potential difference is produced in a region H on the heating resistor 165 that is disposed between two adjacent projections 163c and which has a grounded individual electrode 164 that intersects therewith. Electricity flows through the region H and generates heat therein, thereby allowing a one dot (one pixel) image to be printed on a thermo-sensitive type of recording paper.
However, in the thermal printhead noted above, it is well known that the temperature distribution on each region H will not be uniform when heated up. In other words, it is well known that the central portions between the individual electrode 164 and the adjacent projections 163c will be the hottest, and that the temperature will drop as one moves away from the central portions. The temperature difference between the ends of each region H and the central portions thereof will be more pronounced as the size of each region H increases. This creates a problem in that when using a two-color thermo-sensitive paper as a recording medium for thermal printheads, it is easy to produce colors other than those intended.
For example, when one uses a black/red two-color thermo-sensitive paper and attempts to print a one dot image that is entirely black thereon, the temperature differentials on each region H on the heating resistor 165 causes a large red image to be printed around the periphery of the black image. Note that a two-color thermo-sensitive paper used with thermal printheads is paper which turns a first color (e.g., black) when at or above a predetermined temperature, and turns a second color (e.g., red) at a temperature lower than the predetermined temperature.
Furthermore, increasing the temperature of not only the central portions of each region H but the end portions and their vicinity to a predetermined temperature suitable for printing tends to require a great deal of electrical energy, and thus a great deal of electrical power will be consumed by the thermal printhead. Reducing the size of each region H is thought to be one means for controlling the amount of electrical power consumed. This is because the amount of energy needed can be reduced if the size of the regions H are reduced, even though the regions H are heated up to the predetermined temperature.
However, in the thermal printhead shown in FIG. 16, all of the regions along the length of the heating resistor 165 are capable of generating heat except for both ends thereof. Because of this, when one attempts to reduce the width of the regions H by reducing the array pitch W of the projections 163c of the common electrode 163, it will be necessary to increase the total number of projections 163c and individual electrodes 164. Thus, although the print image resolution will increase, the task of forming the patterns for the common electrode 163 and the individual electrodes 164 will be more difficult. Furthermore, the number of drive IC chips for applying a voltage to the individual electrodes 164 must be increased, and thus the cost of manufacturing the thermal printhead will increase. In addition, if the size of one dot is reduced, the heat capacity of each region H will be reduced, and thus there is a concern that the temperature of each region H will increase higher than necessary when that region is heated up, due to the effect of the temperature of other regions H adjacent thereto, and that thermal degradation will occur.
An object of the present invention is to provide technology for preventing undesirable colors from being generated around the periphery of a desired color even when a two-color thermo-sensitive paper is used as a recording medium.
Another object of the present invention is to make uniform the temperature distribution of a region corresponding to one dot of a heating resistor employed in a thermal printhead.
Another object of the present invention is to reduce the amount of electrical power consumed by a thermal printhead while preventing the manufacturing costs thereof to increase.
In order to solve the aforementioned problems, a thermal printhead according to the present invention is employed to print images on a thermo-sensitive paper. According to a first aspect of the present invention, a thermal printhead is comprised of:
a substrate;
a belt shaped heat resistor disposed on top of the substrate;
a common electrode having a common line disposed along a longitudinal direction of the heat resistor, and a plurality of projections that each project from the common line and which intersect with and electrically connect to the heat resistor; and
a plurality of individual electrodes that are each disposed between mutually adjacent projections which intersect with and electrically connect to the heat resistor.
In this thermal printhead, when an electric potential difference is produced between the individual electrodes and projections of the common electrode, a plurality of regions that have a substantially fixed electric potential (fixed electric potential regions) are formed on the heating resistor in positions in which at least one variable electrical potential region is disposed therebetween. Here, the variable electric potential regions are regions on the heating resistor that are disposed between adjacent pairs of projections and which have an individual electrode that intersects therewith.
The common electrode may be any shape that allows a fixed electric potential regions to be produced on the heating resistor when a voltage is applied between the common electrode and an individual electrode. For example, a fixed electric potential region can be formed between two projections on the heating resistor. In another example, wide projections can be formed, and the portions thereof that are in contact with the heating resistor can form the fixed electric potential regions.
According to another aspect of the present invention, the temperatures of the variable electric potential regions are higher than the fixed electric potential regions when an electric potential difference is produced between the common electrode and the individual electrodes.
An electric potential difference is produced on both ends of each variable electric potential region when an electric potential difference is produced between the common electrode and the individual electrodes. Due to this electric potential difference, electricity flows to the variable electric potential regions and heats these regions up, and the variable electric potential region will become a heat generating region. On the other hand, electricity does not flow to the fixed electric potential regions, and thus these regions will become non-heat generating regions.
According to another aspect of the present invention, the fixed electric potential regions are sized such that a variable electric potential region adjacent to one side of each fixed electric potential region does not have a substantial impact on the temperature of the variable electric potential region on the other side of each fixed electric potential region.
By providing each fixed electric potential region with sufficient width, it will be difficult for the variable electric potential regions to affect each others temperature. Thus, the variable electric regions can be prevented from becoming too hot.
According to another aspect of the present invention, the thermo-sensitive paper is a two-color thermo-sensitive paper that produces a first color at a temperature Tp1 or above, and produces a second color between a temperature Tp2 and the temperature Tp1 (Tp2 less than Tp1). Here, the size of the variable electric potential regions are set such that the entire area of each has the temperature Tp1 or above.
For example, when a two-color thermo-sensitive paper the produces black and red colors is used a recording medium, the variable electric potential regions are sized such that the entire area of each region is at or above a temperature that produces a black color. Note that in this case, it is preferable that the size of each fixed electric potential region is set such that the temperature of each does not substantially exceed Tp2. There will be a change in temperature between the variable electric potential regions and the fixed electric potential regions, and thus a red image can be prevented from being generated around the periphery of a black image.
According to another aspect of the present invention, the portions of the heating resistor that are disposed between two adjacent projections are fixed electric potential regions.
Shaping the common electrode such that portions of the heating resistor are disposed between two adjacent projections is one way of shaping the common electrode to generate fixed electric potential regions.
According to another aspect of the present invention, a bridge connects the two projections on either side of each fixed electric potential region.
A number of benefits can be obtained by forming a bridge between the two projections on either side of each fixed electric potential region. For example, even in situations in which one projection is broken near its base, a fixed electric potential region can still be formed if the tip of the broken portion is connected to the other projection via a bridge.
According to another aspect of the present invention, the portions of the heating resistor that are in electrical contact with the projections of the common electrode are the fixed electric potential regions.
The portions of the heating resistor that are in electrical contact with the projections will become the fixed electric potential regions if the projections have a certain width. Thus, electrical resistance will be reduced, and the formation of the common electrode pattern will be simplified, because the projections have a wide width.
According to another aspect of the present invention, each projection on the common electrode is in electrical contact with the heating resistor at a plurality of points thereon, and the portions of the heating resistor which are interposed between the plurality of points are the fixed electric potential regions.
For example, each projection can be brought into contact with the heating resistor at a plurality of points thereon by branching the projection or by making each projection non-linear.
According to another aspect of the present invention, the tip of each projection on the common electrode is branched. Here, each branch of each projection intersects with and is electrical contact with the heating resistor, and the portions of the heating resistor that are interposed between the branches are fixed electric potential regions.
Compared to a configuration in which narrow projections project from the common line and contact with the heating resistor, forming a relatively thick projection and then branching out narrow branches that contact with the heating resistor is advantageous because it is easier to form the common electrode and electrical resistance will be reduced.
According to another aspect of the present invention, bridges are formed to connect the branches that sandwich each fixed electric potential region.
Similar to that noted above, a fixed electric potential region can be formed even in situations in which one branch is broken at its base.
According to another aspect of the present invention, each projection on the common electrode is non-linear, and intersects with and is in electrical contact with the heating resistor at a plurality of points thereon. Here, the portions of the heating resistor that are interposed between the plurality of points on the projections are the fixed electric potential regions.
Because the projections are non-linear in shape, it will be difficult for the heat of the variable electric potential regions to escape to the common electrode, thus allowing one to control the power consumption of the thermal printhead.
According to another aspect of the present invention, the individual electrodes are branched. Here, the branches of each individual electrode are disposed such that one projection is interposed therebetween, and intersect with and electrically contact with the heating resistor.
Because a plurality of variable electric potential regions that each correspond to one dot can be formed on the heating resistor, it becomes easy to make the temperature of each of these regions uniform.
According to another aspect of the present invention, a thermal printhead is comprised of:
a substrate;
a belt shaped heat resistor disposed on top of the substrate:
a common electrode having a common line disposed along a longitudinal direction of the heat resistor, and a plurality of projections that each project from the common line and which intersect with and electrically contact with the heat resistor; and
a plurality of individual electrodes that are each disposed between mutually adjacent projections and which intersect with and electrically contact with the heat resistor.
Here, a plurality of regions on the heat resistor are comprised of a plurality of heat generating regions that are electrically connected to the Individual electrodes, and at least one non-heat generating region disposed between each pair of the plurality of heat generating regions and not electrically connected to the individual electrodes.
According to another aspect of the present Invention, the plurality of projections are spaced apart from each other at a predetermined distance, and the plurality of heat generating regions and the plurality of non-heat generating regions are arranged on the heating resistor such that one heat generating regions alternates with one non-heat generating region.
According to another aspect of the present Invention, the heat generating regions and the non-heat generating regions have different widths.
According to another aspect of the present invention, the plurality of heat generating regions and the plurality of non-heat generating regions are arranged on the heating resistor such that one heat generating region alternates with a plurality of non-heat generating regions.
According to another aspect of the present invention, a thermal printhead is comprised of:
a substrate:
a belt shaped heat resistor disposed on top of the substrate;
a common electrode having a common line disposed along a longitudinal direction of the heat resistor, and a plurality of projections that each project from the common line and which intersect with and electrically contact with the heat resistor; and
a plurality of individual electrodes that are each disposed between mutually adjacent projections and which intersect with and electrically contact with the heat resistor, each individual electrode comprising a first end, a second end, and a terminal disposed on the second end.
Here, the total number of terminals is less than the total number of first ends, and each terminal is electrically connected to two or more first ends that are electrically connected to mutually adjacent regions on the heat resistor.
According to another aspect of the present invention, a plurality of first ends branch out from each individual electrode.
According to another aspect of the present invention, mutually adjacent groups of the plurality of individual electrodes share one terminal.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.